The present invention relates to a system for controlling the static pressure within a controlled space and, more particularly, to a system which senses the amount of air moving through a hole in the wall in the controlled space for controlling the static pressure therein.
In many applications, it is desirable for controlling the static pressure within a controlled space. For example, in the so called "clean rooms" used, for example, in semiconductor processes and various kinds of laboratories, it is necessary to keep the static pressure within the controlled space above the static pressure of the area outside of the controlled space. This will ensure that contaminants from outside of the controlled space will not migrate into the controlled space. In other applications, such as experiments in laboratories which may produce harmful contaminants, it may be necessary to confine these contaminants within the controlled space. Thus, the static pressure within the controlled space is controlled at a point below the static pressure outside of the controlled space to ensure that such contaminants will not migrate outside of the controlled space.
The prior art has raised or lowered the static pressure within a space either above or below the static pressure outside of the space by placing a fan either in the supply duct to the space or in the exhaust duct. However, such systems did not control the static pressure within the space. Thus, such systems often wasted energy because the fans had to be continually operated at a point to ensure that, during extreme conditions, the space static pressure was either above or below the static pressure outside of the space, as desired. On the other hand, it has been known to utilize static pressure sensors for directly sensing the static pressure within a space to control the amount of air either supplied to the space or exhausted therefrom. Such systems, however, tended to be complex and, therefore, expensive and less reliable. Also, static pressure does not follow a linear relationship with air volume.
The use of a velocity sensor in this type of application has the advantages of being able to control accurately air flows having much smaller values than those for which static pressure sensors can be used, to provide a linear relationship between the velocity of air being sensed and the amount of air being controlled, and to provide a simpler overall system.
It has been known to use velocity sensors for controlling the pressure differential between the interior and exterior of a building. Such prior art systems, however, have at least two disadvantages. First, the velocity sensor itself does not know which way air is passing through it. Thus, it may be possible, as a result of sudden changes in conditions, such as a sudden reversal in air flow moving through the sensor or under start up conditions, that the velocity sensor will control the static pressure within the building at a point negative to the point at which it is designed to control the static pressure. Moreover, unless the air moving through the supply or exhaust duct is also sensed, any changes in static pressure within the duct will not reach the velocity sensor mounted in the wall of the building without a time lag which depends upon the location of the velocity sensor with respect to the duct and the size of the controlled space. This time lag can be substantially reduced by adding a second air flow sensor within the duct itself.